Microstructural determination of fast diffusing species in thin film diffusion couples

A transmission electron microscope was used to study the room temperature interdiffusion of Pd/Sn thin film couples. It was found that the interdiffusion proceeds by the rapid diffusion of palladium into tin, followed by the formation of the intermetallic compound PdSn 4. The microstructure of PdSn 4 shows that the compound is formed inside the original large tin grains. Furthermore, the solid state structural transformation of tin into PdSn 4 involves only the positioning of palladium atoms in the interstitial sites together with the associated faultings and thus can take place readily without extensive modifications to the original tin structure. In the Pd/Sn diffusion couple, therefore, palladium is the faster diffuser. The detection of such a simple structural transformation can in principle permit the determination of the direction of net atomic flow which in turn provides information on the relative magnitude of the diffusivities of the components in thin film diffusion couples.